U.S. Pat. Nos. 4,578,990 and 4,627,271 describe a differential pressure capillary viscometer which may be used to measure viscosity independent of flow rate and temperature fluctuations. These patents disclose a viscometer in which a solvent is pumped from a reservoir into a system comprising a solute injection valve upstream of two capillary tubes which are separated by a large depository column which is used to trap solute, so that only solvent flows through the second capillary tube. Changes in pressure across both capillary tubes are measured and converted into electrical signals, which are fed to a differential logarithmic amplifier. The output signal of the differential logarithmic amplifier is related to the natural logarithm of the relative viscosity .eta..sub.r. Both the inherent and intrinsic viscosities may be related mathematically to the experimentally measured value for the relative viscosity. Although the apparatus disclosed in these patents provides a viscosity measurement which is independent of flow rate and temperature fluctuations, there is a danger that after multiple uses the solute which has been retained by the large depository column, may begin to elute and affect the pressure changes in the second capillary tube, thereby leading to inaccuracies in the viscosity measurement.
Accordingly, there is a need in the art for a differential pressure capillary viscometer which is not only capable of measuring viscosity independent of flow rate and temperature fluctuation, but which is not subject to the potential for inaccuracy which is inherent in the prior art device discussed above.